ESP8266-HDK-dat.md

chip info

pin	name	GPIO	function	Note	note
25	U0_RXD	3	IO3		I/O UART Rx during flash programming
26	U0_TXD	1	IO1	SPI_CS1	I/O UART TX during flash programming;
14	IO2	2	U1_TXD		Flash debug output
13	MTDO	15	U0_RTS	HSPI_CS
18~23	SDIO
8	XPD_DCDC	IO16			Deep-sleep wakeup (need to be connected to EXT_RSTB); GPIO16

pin	default boot	Against Mode	ESP-12F-dat	ESP-12S-dat	func
RESET	pull up + cap 0.1 UF	Reboot	ext.	int.
EN	pull up	Reboot	ext.	int.
IO0	pull up	Flash Mode	ext.	int.	Boot mode control
IO2	pull up		pull-up w/LED	pull-up w/LED	on-module LED
GPIO15	pull down	Fail into SDIO Mode	ext.	int.	MTDO

(4. Peripheral Interface)

Hardware Design - The input voltage range is 0 to 1.0 V when TOUT is connected to external circuit.

The ESP8266EX schematics include seven aspects:

• Power supply • Power-on sequence and reset • Flash • Crystal oscillator • RF • External resistor • UART

Feature	Support	Notes
PWM (analogWrite)	❌ No	GPIO16 does not support hardware PWM.
Interrupts (attachInterrupt)	❌ No	Cannot be used for interrupts.
I²C / SPI / UART	❌ No	Not multiplexed for any peripheral functions.
Analog input	❌ No	ESP8266 has only one ADC (ADC0), not on GPIO16.
Open-drain / Pull-up/down	❌ No	Limited internal resistor configuration.
Output during deep sleep	⚠️ Caution	When used for deep sleep wake, it must not be driven externally.

Feature	Support	Notes
Digital output	✅ Yes	Use with `digitalWrite()`.
Digital input	✅ Yes	Use with `digitalRead()`.
Deep sleep wake	✅ Yes	Connect GPIO16 to RST to enable timed wake from deep sleep.

GPIO	Usable for Interrupts?	Notes
GPIO0	✅ Yes	Must be HIGH at boot (pulled LOW enters flash mode)
GPIO2	✅ Yes	Must be HIGH at boot
GPIO4	✅ Yes	Safe
GPIO5	✅ Yes	Safe
GPIO12	✅ Yes	Safe
GPIO13	✅ Yes	Safe
GPIO14	✅ Yes	Safe